Fabrication and surface characterization of pulsed reactive closed-field unbalanced magnetron sputtered amorphous silicon nitride films

Amorphous silicon nitride (a-SiNx) thin films were fabricated by pulsed reactive closed-field unbalanced magnetron sputtering ultra-high purity n-type single crystal silicon in Ar–N2 mixtures. The effect of N2 fraction on the discharge behavior, deposition rate, composition, chemical bonding configurations, surface morphology, nano-hardness, elastic modulus and optical band gap were primarily investigated. It was found that good stability of reactive sputtering process was maintained by the adoption of the bipolar pulsed magnetron power supply with 180° out-of-phase and the pulsed substrate bias. The arc events and the disappearing anode effect in DC reactive magnetron sputtering of a-SiNx films were prevented. A higher deposition rate (>20 nm/min) for a-SiNx films was achieved. The radical transition of the sputtering mode leads to the deposition rate initially decreased dramatically and then slowly with the increase of N2 fraction. N to Si atomic ratio (N/Si) gradually increased and N is preferentially incorporated in its NSi3 stoichiometric ratio configuration and the Si–N network followed a tendency to tetrahedral chemical order with the increase of N2 fraction in the inlet gases. The a-SiNx films deposited at high N2 fraction were consistently N-rich. The film surface microstructure was transformed from coarse granular mounds surrounded by tiny micro-void regions to homogeneous, continuous, dense and slender hills, as well as a progressive densification and refinement of the film microstructure occurs as the N2 fraction is increased. With the increase of the N/Si atomic ratio, the resistance of the a-SiNx films surface region to plastic deformation improved; hardness and elastic modulus increased, correspondingly. The as-sputtered a-SiNx films exhibit good optical transparency in visible region and the optical band gap Eopt can be varied in a broad range of 1.70–3.62 eV depending on the N2 fraction in Ar–N2 mixtures. The increment of optical band gap Eopt of the as-sputtered a-SiNx films is determined primarily by the recession of the valence band maximum.